1. Field of the Invention
The present invention relates to an optical receiving apparatus, and particularly to an optical receiving apparatus for receiving an optical differential quaternary phase shift keying (DQPSK) signal in an optical fiber communication system, and more particularly to an optical receiving apparatus which can control optical characteristics of two optical interferometers installed in the inside of the optical receiving apparatus to provide the optimum state for reception.
2. Description of the Related Application
A present optical communication system uses a binary modulation and demodulation technologies using light intensity. Specifically, a transmission side converts “0” and “1” of digital information into ON and OFF of light intensity, and transmits it to an optical fiber. A reception side photoelectric-converts the light propagating through the optical fiber and restores the original information. In recent years, with remarkable popularization of the Internet, the transmission capacity requested to the optical communication system is extremely increased. The increase of the transmission capacity is dealt with by raising the speed of ON and OFF of light, that is, the modulation speed. However, in general, the method of realizing the large capacity by raising the modulation speed has matters as described below.
First, in order to turn on and off the light, a new electric device and a new optical device capable of performing at ultra-high speed are required. There is a matter that cost and time are required to develop the new devices. There is also a matter that when the modulation speed is raised, the transmittable distance limited by the chromatic dispersion of an optical fiber becomes short. In general, when a bit rate is doubled, the transmission distance limited by the chromatic dispersion becomes ¼. Similarly, there is also a matter that when the modulation speed is raised, the transmittable distance limited by the polarization mode dispersion of an optical fiber becomes short. In general, when the bit rate is doubled, the transmission distance limited by the polarization mode dispersion becomes ½.
Then, recently, as an optical modulation and demodulation system to increase the transmission capacity, modulation and demodulation systems using the phase of light, not the related art binary modulation of the light intensity, are studied. Among them, quaternary phase shift keying (QPSK) particularly receives attention since it has following features. That is, in the QPSK, since a symbol rate is half a bit rate, the ultra-high speed electric device or the optical device operating at the bit rate, which is required in the related art binary modulation of light intensity, is not required. Besides, in the case of the QPSK, the communication distance limited by the chromatic dispersion of the optical fiber can be extended four times longer than that of the related art binary modulation system of the light intensity, and the communication distance limited by the polarization mode dispersion is extended two times longer than that of the binary modulation system of the light intensity. Thus the QPSK has also a feature that it is suitable for long distance communication. Incidentally, a specific modulation and demodulation system of the QPSK is disclosed in patent document 1.
Among the QPSK, differential quaternary phase shift keying (DQPSK) using differential coding has a simple receiver structure and therefore, it receives attention. In the DQPSK signal, the phase of light is made to correspond to π/4, 3π/4, 5π/4 or 7π/4 correspondingly to a set of two differentially coded bits, that is, (0,0), (1,0), (1,1) or (0,1). At the reception side, the optical DQPSK signal transmitting through the optical fiber is branched into two, and in order to perform differential decoding, the signals are made to pass through two optical interferometers, which are set so that a delay-time difference between two arms in each of the interferometers becomes a 1-symbol time of the DQPSK, and are converted into intensity signals, and these are converted into electric signals and are received. Besides, the respective phase differences between both the arms of the two optical interferometers are set to +π/4 and −π/4. As compared with the receiver of the QPSK, the receiver of the DQPSK has features that a local oscillator light source is not required, and it is not necessary to adopt a polarization-diversity-receiver configuration, and the number of receivers is halved. As a result, a low cost optical receiving apparatus can be realized.
As stated above, in the receiver of the optical DQPSK signal, it is necessary to set the phase differences between both the arms of the optical interferometers to +π/4 and −π/4. However, in the optical interferometer, the phase difference is shifted from ±π/4 due to the ambient temperature or aging of parts constituting the interferometer. This phase shift causes deterioration of reception sensitivity. For example, according to non-patent document 1, when the phase difference is shifted from the optimum phase difference by 6 degrees (=π/30), the sensitivity is deteriorated by 1 dB. In a generally used optical interferometer, by the temperature dependency of refractive index of a material forming an optical waveguide and the temperature dependency determined by the linear expansion coefficient of an optical waveguide substrate, when the temperature rises, the optical path length is extended, and when the temperature is lowered, the optical path length is shortened. Accordingly, the phase difference of two arms of the optical interferometer becomes large by the rise of temperature, and the phase difference becomes small by the lowering of temperature. With respect to a structure in which a heater is attached to an optical interferometer used in a receiver of an optical DQPSK signal, and a difference between optical path lengths of two arms is adjusted to set a phase difference to a desired value, an example thereof is disclosed in, for example, non-patent document 2.
That the phase difference is sensitive to the temperature means that there can occur a case where a once set phase difference is shifted from the optimum value by the influence of ambient temperature. Further, by aging of the heater or the like, a desired amount of heat comes not to be generated with the lapse of time, and consequently, the phase difference is shifted from the optimum value with the lapse of time.
As a technique to control and to minimize the shift of the phase difference from the optimum value, there is a technique disclosed in, for example, patent document 2, patent document 3 or patent document 4.
In the technique of the patent document 2, for example, an eye opening monitor is provided in a receiver, an average value, a minimum value or a maximum value of a distribution of signal amplitude is obtained, and the phase of an arm at one side of an optical interferometer is controlled so that the average value becomes maximum, the maximum value becomes minimum, or the minimum value becomes maximum. In the technique of the patent document 3, for example, a preamplifier output is squared (or full rectified), and the phase of the optical interferometer is controlled so that the low frequency component of the signal becomes minimum. In the technique of the patent document 4, for example, preamplifier outputs of I and Q branches are respectively multiplied by outputs of data regenerators of the Q and I branches, and the low frequency signal component is used to control the optical interferometer.    [Patent document 1] JP-T-2004-516743    [Patent document 2] JP-A-2007-181171    [Patent document 3] JP-A-2007-60583    [Patent document 4] JP-A-2007-20138    [Non-patent document 1] Keang-Po Ho, “The Effect of Interferometer Phase Error on Direct-Detection DPSK and DQPSK Signals”, IEEE Photonics Technology Letters, Vol. 16, No. 1, 308 (2004)    [Non-patent document 2] Yannick Keith Lize, et al, “Phase-Tunable Low-Loss S-, C-, and L-Band DPSK and DQPSK Demodulator”, IEEE Photonics Technology Letters, Vol. 19, No. 23, 1886 (2007)